1877.] 



Excitability of Motor Nerves. 



215 



strength of a single Grove's cell caused the excitability of the nerve 

 towards the breaking excitation, supplied through the same electrodes by 

 a small Daniell's cell, to rise from 5000 to 100,000 Ohms. Similarly a 

 strong induction-shock supplied by a single Grove's cell with the secondary 

 coil at zero, and thrown in between the electrodes from a small Daniell's 

 cell, caused the excitability of the nerve towards a closing stimulus sup- 

 plied by the latter to rise from 40,000 to 185,000 Ohms. In conducting 

 these experiments, I was not able to perceive that the direction of the 

 strong or injuring current made any difference in the nature of the 

 results. 



§ 4. This concludes my observations so far as stimuli of minimal 

 intensity are concerned ; and, at the suggestion of Dr. Burdon-San- 

 derson, I terminated this inquiry regarding the electrotonic condition 

 of injured nerves by substituting for voltaic stimuli of minimal 

 intensity, voltaic stimuli of minimal duration. The method which 

 I employed in this part of the research was as follows : — The frog 

 (JR. temporaria) having been prepared as already described in § 1, 

 the duration of the voltaic stimulus was graduated by means of a 

 heavy pendulum, which constituted one pole of the battery, and which, 

 while swinging, made contact at the lowest point in its arc with the other 

 pole. The latter consisted of a fixed platinum wire placed vertically, 

 and the contact was made with it by means of a pointed piece of 

 metal attached to the moving pole and placed horizontally. Thus by 

 increasing or diminishing the distance through which the pendulum, 

 or moving pole, was allowed to swing, a stimulus of any required 

 duration could be supplied to the muscle interposed in the circuit. 

 As a battery I employed a single Daniell's cell ; and, lastly, I inter- 

 posed a rheochord, a commutator, and a key. Such being the apparatus, 

 the course of any one experiment was very simple. By means of the 

 swinging pendulum, the uncut muscle was supplied with a stimulus of 

 measured duration, which was then graduated down to the point at which 

 the break of the current succeeded the make with a rapidity just suffi- 

 ciently great to prevent the muscle from responding to either stimulus, 



(a) when the femoral end rested on the anode, and (6) when this end 

 rested on the kathode. These two durations having been noted, the 

 nerve was cut through at the usual place, and the observations (a) and 



(b) repeated as rapidly as possible. It was invariably f oimd that in both 

 cases a much shorter duration of the voltaic stimulus was required to 

 produce minimal stimulation than had been required to do so before the 

 nerve was cut, the intensity of the voltaic current, of course, being kept 

 uniform throughout. 



An apparent objection to this method of experimenting is apt to sug- 

 gest itself, viz. that the make and the break must follow one another 

 much too rapidly to admit of the observer being able to eliminate the 

 effects of the former from those of the latter stimulus. But, as a matter 



